Our work over the past five years has established the observation that postmenopausal women with primary hyperparathyroidism show a relative preservation of cancellous bone as compared to age-matched post menopausal women without primary hyperparathyroidism. The well-known fact that postmenopausal women are at risk for cancellous bone loss raises important questions about the interactions of the hyperparathyroid state with the post menopausal state in women who develop primary hyperparathyroidism in their postmenopausal years. In the renewal period, further investigation of this observation will include five specific aims: 1. to describe the evolution and reversibility of the hyperparathyroid process in the skeleton of postmenopausal women with primary hyperparathyroidism: longitudinal analysis by bone mineral densitometry, percutaneous bone biopsy, serum and urinary markers of bone turnover. 2. to determine the influence of primary hyperparathyroidism on bone loss in postmenopausal women. 3. to determine by histomorphometric analysis the mechanism of cancellous bone preservation and cortical bone loss in postmenopausal women with primary hyperparathyroidism. 4. to determine the utility of non-invasive markers of bone turnover as indicators of processes in hyperparathyroid bone. 5. to evaluate the effects of estrogen therapy on bone mineral density, histomorphometric parameters of bone, and biochemical markers in postmenopausal women with primary hyperparathyroidism.